To easily perform the sealing action and exchange of the sealing ring of the plug/twist connection part of a medical or dental treatment instrument.

At the sealing ring of the plug/twist connection part of this medical or dental treatment instrument, the sealing ring 23 for sealing the radial direction medium pipe line part 21 of a boundary part 19 is arranged on the cross section of the radial direction medium pipe line part 21 and seated to an annular groove. The sealing ring 23 and the annular groove are provided with a width larger than the cross section dimension of the radial direction medium pipe line part 21 and the sealing ring 23 is provided with respective annular sealing elements on both sides of the radial direction medium pipe line part 21 and at least one through-duct passed through in a radial direction in the intermediate area.